Apparatus for Drying Clothes or Other Solids Using Microwave Energy Under Reduced Pressure with Energy Recovery While Avoiding Arcing

ABSTRACT

An apparatus for drying clothes or other solids that includes a rotating drum with microwave generator inside and rotating piping connections for energy recovery via drum jacket while sealing microwaves and pressure (vacuum), and selectable controls that provide conventional heating at a preset drum outlet moisture content, or not, depending upon the potential for metal in the load, to avoid arcing.

BACKGROUND Prior Art

The following is a tabulation of some prior art that presently appears relevant:

U.S. Patent Number Issue Date Patentee 3,410,116 1968 Nov. 12 Levinson 3,439,431 1969 Apr. 22 Heidtmann 3,854,219 1974 Dec. 17 Staats 4,057,907 1977 Nov. 15 Rapino, et al. 4,250,628 1981 Feb. 17 Smith, et al. 4,334,136 1982 Jun. 8 Mahan, et al. 4,356,640 1982 Nov. 2 Jansson 5,321,897 1984 Jun. 21 Holst 4,490,923 1985 Jan. 1 Thomas 4,510,361 1985 Apr. 19 Mahan 4,523,387 1985 Jun. 18 Mahan 4,703,565 1987 Nov. 3 Kantor 4,765,066 1988 Aug. 23 Yoon 4,829,679 1989 May 16 O'Connor, et al. 4,856,203 1989 Aug. 15 Wennerstrum 5,187,879 1993 Feb. 23 Holst 5,270,509 1993 Dec. 14 Gerling 5,315,765 1994 May 31 Holst 5,321,897 1994 Jun. 21 Holst 5,341,576 1994 Aug. 30 Tsutomo, et al. 5,396,715 1995 Mar. 14 Smith 7,665,226 2010 Feb. 23 Tsurata, et al. 8,015,726 2011 Sep. 13 Carow, et al.

BACKGROUND Prior Art

Inventions applying microwave energy to drying of solids have been around nearly since the residential microwave oven entered the marketplace in the mid 1960's. For example, in U.S. Pat. No. 3,410,116 (1968) to Levinson a device is described that could be used as a microwave oven or converted to other uses, including drying clothes. The source of drying energy was a microwave generator, though not described in detail. However, drying of household laundry using microwave energy has some inherent problems that were not addressed in this early patent and, perhaps, not completely addressed since. One of the most difficult issues is that of arcing that can occur between metal contained in clothing (zippers, rivets, buttons, coins, etc.) and a grounded surface of the dryer. Microwaves are nearly totally reflected by most metal surfaces, but some skin penetration does occur with energy transfer to the metal. That energy releases electrons in the metal. According to classical electromagnetic theory, a free charge can exist only on the surface of a conductor, as the electric field within the metal must remain zero. Accordingly, free electrons will distribute as a surface charge and in such a way as to provide the appearance of a uniform electric field emanating from the surface when viewed at a great distance (infinity). If that electric field is represented by lines perpendicular to the surface and directed outward, those lines for a flat surface will be equally spaced. At a curved surface, the charge will aggregate so that the field lines will be closely spaced at the surface but will diverge and appear equally spaced at a distance. The density of the surface charge will be inversely proportional to the radius of curvature. This means that sharp points on the metal surface can create a very high electric field when viewed close to that surface. It is possible that the electric field could exceed the breakdown voltage of air, causing the air molecules to ionize and become conductive. An arc results from the transfer of electrons from the metal surface to ground. The energy associated with that arc can create high temperatures that can damage associated objects. In a later attempt to control arcing, in U.S. Pat. No. 4,523,387 (1985) to Mahan, a non-conductive liner was added to the drum. In the present invention, the drum surface must remain conductive to facilitate heat transfer from the jacket. In U.S. Pat. No. 5,270,509 (1993) to Gerling, an electronic system to monitor the electric field in the drum was added which could interpret a sudden drop in that field strength as arcing, and then reduce the microwave power. In U.S. Pat. No. 5,396,715 (1995) to Smith a fire suppression system is included triggered by a flame sensor. Smith makes the statement that thermal damage to fabric is the result of rapid heating of metal objects by microwaves. Rather, the damage is the result of the arc that may occur between the metal object and some lower potential, likely the grounded drum. However, some materials do readily absorb microwaves (some plastics, some liquids, even some crystals like graphite and doped silicon) and should such materials be included with clothes burning of fabric could occur. Checking pockets will always be part of successful microwave drying. Still, arcing remains a problem which inhibits the application of microwave drying to residential laundry.

According to Paschen's Law, the breakdown voltage for air will initially decline as the pressure is lowered from normal atmospheric pressure, reaching a minimum as the pressure is continuously reduced. For a vacuum dryer operating in that region of the Paschen Curve with negative slope, arcing in the dryer may be an even greater problem. A dryer operating at reduced pressure must expect a greater incidence of arcing. If, as suggested, the potential difference that causes arcing results from a building of surface charge on metal objects, a mechanism that conveys that charge to ground could avoid arcing. Such a mechanism exists naturally with saturated fabrics. Liquid water, except when extremely pure, has the ability to conduct surface charge to ground. However, as the surface of the fabric dries, even though water still exists inside the fibers, that conductivity will be gone. So, at some point during the drying process, metal surface charge will again build. If microwave energy is replaced at that point with conventional heating, some overall efficiency is lost, but that loss can be minimized with good sorting of loads.

One reason to apply microwave energy to drying is to provide higher energy efficiency than the conventional method of hot air drying. Microwaves will be absorbed by water molecules without being significantly absorbed by the surrounding air. Air may still be used to convey the vaporized water from the dryer, but the energy associated with that air will not represent wasted energy. It is also possible to minimize the temperature rise of the solid material using microwaves. If there is no air movement, liquid water will only be removed by raising the water temperature to the point where its vapor pressure is equal to the atmospheric pressure; i.e., to its atmospheric boiling point. However, if the incoming air is relatively dry, the liquid water can be removed at a lower temperature by contacting the water with moving air. The driving force to transfer water molecules from the liquid surface to the air is proportional to the vapor pressure of the water, which in turn is determined by temperature. The higher the water temperature, the faster the water will be removed. The same is true for a higher air flow rate, and for better contact between solids and air. It occurred to some that reduced pressure drying of solids using microwaves has the potential to further reduce the drying temperature by increasing the driving force for a given temperature. That is, for the same temperature, the partial pressure of water vapor will be greater at reduced total pressure, improving the rate of transfer of water molecules to the air. For example, U.S. Pat. No. 4,250,628 (1981) to Smith and Uthe describes a method and apparatus for drying fabric under vacuum using microwave energy and indirect heating from hot water at the bottom of the drying chamber, which does not rotate. Air from a blower is added to the chamber at the end of the cycle. There is no discussion of the potential for arcing.

A rotating drum is helpful in most drying applications to provide continuously good contact between air and solids which facilitates mass transfer of water vapor to air and removal from the dryer. However, a rotating drum presents sealing problems with both microwaves and air, and especially so if the drum operates under vacuum. For example, U.S. Pat. No. 4,765,066 (1988) to Yoon describes an elaborate non-contact sealing system to allow for drum rotation without leaking microwaves by controlling clearances at a small fraction of the energy wavelength. The unit did not operate under vacuum.

All similar inventions heretofore known fail to take full advantage of the minimum temperature and energy efficiency possible with microwave drying by combining a rotating drum, reduced pressure, and energy recovery (including waste heat from the microwave generator), while effectively sealing both air and microwaves and avoiding arcing.

SUMMARY

In accordance with one embodiment a sealed drum, rotating on a horizontal axis, loaded with clothes or other fabrics, is supplied with outside air and irradiated with microwave energy from a generator that rotates with and is located inside the drum, under partial vacuum. Air exits the dryer through a filter and flows through a vacuum pump which returns this moisture-laden air to the outer jacket of the drum and finally the air is directed to a trapped and vented drain. If the load may contain metal objects, then, when a moisture sensor in the vacuum pump discharge detects a preset reduced moisture level, the microwave generator is turned off and an electric heater turned on to raise the temperature of incoming air and continue the drying process without allowing arcing from any metal objects in the fabric. Unlike microwave energy, this heated air will transfer energy to the fabric as well, but the temperature will at least be limited by the saturation temperature of water at reduced pressure. When a further reduced moisture level is detected by the moisture sensor, the drying process is complete and all power to the apparatus—heater, vacuum pump, and rotating mechanism—is shut down. Air is directed to and from the rotating drum via dual flow rotating unions and through a double pipe which also serves as the axle for the drum. Inside each rotary union, the outer pipe is sealed via single mechanical seal while the inner pipe is sealed via stuffing box (although a mechanical seal is possible here also). When the load contains no metal, as with sheets, towels, etc., and for other solids, a selectable switch will allow microwave energy to continue drying until the final moisture level is reached.

Advantages

In this embodiment, the continuous rotation of the drum provides good contact between fabric and air, the low absolute pressure inside the drum minimizes the temperature required to vaporize the water, the microwave energy targets water molecules while the energy normally wasted from the microwave generator is utilized to enhance drying, and the energy represented by the moisture laden air that leaves the drum is partially recovered via the outside heat transfer jacket of the drum, along with some of the heat of compression supplied by the vacuum pump. Using low voltage rotary contacts, the microwave source has been mounted inside the rotating drum. Most of the energy associated with the inefficiency of converting house current to microwaves is taken up by the incoming air and then transferred by convection to the solids in the dryer. Additional waste energy from the microwave generating equipment is transferred by conduction through the wall of the enclosure to the solids on the outside of that wall. Microwaves are sealed inside the drum by virtue of the small diameter of the orifice(s) for air flow into the drum, the small diameter of orifices in the support for the outlet filter media, and the conductive elastomeric seal and the small dimension of the gap between the form matching, overlapping door and the drum. Some of the latent heat of vaporization of the water and the heat of compression from the vacuum pump are recovered by directing air from the vacuum pump to the dryer jacket. The space between the inner drum and outer shell—the jacket—includes a spiral-wound baffle intended to maintain air velocity to promote heat transfer and prohibit bypassing as it directs the air flow across the drum and to the outlet pipe. As moisture condenses, the air flow rate and direction are intended to keep the liquid flowing so no buildup occurs that would inhibit additional heat transfer and condensation. This two-phase flow exits the dryer and enters a typical trapped and vented sewer line. As clothes are dried, the liquid water in the fabric will initially minimize any buildup of charge on metal objects by its conductivity. At some point the reduced moisture level will inhibit its ability to conduct charge to the grounded drum, at which point the energy source will be switched from microwaves to hot air, for those loads that may contain metal, to complete the drying of fabrics, thus avoiding arcing.

This embodiment provides a combination of low temperature drying and energy conservation and recovery not currently available. However, it should be noted that this embodiment, as with current technology, will provide its maximum energy efficiency when used continuously; that is, with one batch closely following the last. Otherwise, the energy associated with the elevated operating temperature of all dryer parts will be wasted following each batch, lowering the efficiency per batch.

DRAWINGS Figures

FIG. 1 shows a simplified view of the overall rotating drum assembly and the associated air flow.

FIG. 2 shows a cross sectional view of the rotating drum and the air flow direction through the microwave generator enclosure, through the drum and out through the filter.

FIG. 3 shows a more detailed cross sectional view of the microwave generator enclosure.

FIG. 4 shows a top view of the drum access door.

FIG. 5A and FIG. 5B show cross sectional views of the door sealing mechanism.

FIG. 6 shows a simplified electrical wiring diagram for the assembly including controls.

DRAWINGS Reference Numbers

-   -   11 120/240 VAC rotary contacts     -   12 air regulating valve     -   13 dual flow rotary union     -   14 pillow block bearing     -   15 rotating vacuum drum     -   16 lip-sealed drum door     -   17 dry vacuum pump     -   18 drum rotating mechanism     -   19 high voltage capacitor     -   20 high voltage diode     -   21 magnetron tube     -   22 wave guide and microwave antenna cover     -   23 high voltage microwave generator canister     -   24 high voltage transformer     -   25 baffle to provide for load rotation     -   26 air filter     -   27 insulation     -   28 rotating vacuum drum baffled jacket     -   29 canister mounting flange     -   30 canister air flow baffle     -   31 low voltage microwave generator wire connector     -   32 slotted hinge     -   33 tapered lever arm     -   34 locking door handle     -   35 tapered arm guide     -   36 tapered arm shackle     -   37 door lifting handle     -   38 conductive elastomeric seal     -   39 inlet air electric resistance heater     -   40 inlet heater temperature control switch     -   41 minimum vacuum switch     -   42 outlet air (low, low low) moisture switch     -   43 magnetron tube temperature limit switch     -   44 canister critical flow orifice(s)     -   45 manual selector switch     -   46 momentary contact start switch     -   47 drum proximity switch     -   48 momentary contact stop switch

DETAILED DESCRIPTION FIGS. 1, 2, 3, 4, 5A & B, 6 First Embodiment

FIG. 1 provides a simplified external view of all mechanical components of the unit. Arrows in this figure indicate the rotation of the axle and drum assembly—everything between the stationary housings of the dual flow rotary unions 13. The bodies of these unions and the inlet/outlet pipes connected to them, opposite the drum axle, are all stationary components. The drum axle is supported by pillow block bearings 14 on both sides of the drum 15. The bearing housings need to be supported by the frame of the entire unit, but the frame is not shown, nor is any overall enclosure shown, which would surely be part of any appliance sold for residential use. The drive motor and mechanism for rotating the drum 18 are shown, but not in detail. The drum may be belt, chain, or gear driven, and the motor may be fixed or variable speed. The sheave, sprocket, or gear teeth can encircle the drum or be attached to the axle, although that will extend the length of the unit. The drum opening door 16 is depicted as an elliptical shape, but could be any shape. The radius of the door surface that seals to the drum must conform closely to the outside radius of the drum. The drum is shown as cylindrical with flat ends gusseted to the outside pipe of the axle. The ends could be of any formed shape. Although formed ends could be designed for external pressure with thinner metal, the flat end was chosen to simplify the connection and sealing of the generator canister and outlet filter housing.

FIG. 2 shows a cross sectional view of the drum and its internal parts, along with an internal view of the double-pipe axle near the drum. An arrow follows the air flow through the drum. Insulation 27 is shown, which is critical to provide for energy recovery.

FIG. 3 shows a cross sectional view of the canister 23 that houses the high voltage microwave generator components (capacitor 19, high voltage diode 20, magnetron tube 21, magnetron tube temperature limit switch 43, and transformer 24). The air flow baffle 30 is required to direct all air flow across the magnetron tube cooling fins, and then through the pressure determining critical flow orifice(s) 44. The maximum diameter of the orifice must be a small fraction of the wavelength of the microwaves generated. If the area of one such orifice is not sufficient to provide the design air flow rate, then multiple holes must be drilled such that the combined area does provide that flow as the pressure drops from near atmospheric to near the vacuum pump suction (absolute) pressure. The canister flange 29 is intended to attach to the end of the drum (perhaps by welded studs and nuts) and be sealed by a gasket from the vacuum inside the drum, while grounded to the drum through the attachment. A pin type low voltage wire connector 31 provides electrical connection/disconnection for installation and maintenance purposes.

FIG. 4 shows a top view of the drum access door 16. The type of closure shown is merely one that would work. A closure designed for residential use would likely require a more aesthetically pleasing design, replacing details for the hinges 32, tapered lever arm 33, locking door handle 34, tapered arm guide 35, tapered arm shackle 36, and door lifting handle 37, while maintaining function.

FIG. 5A and FIG. 5B show cross sectional views of the access door 16 and the mechanism from FIG. 4 which provides compression of the elastomeric seal 38 material. Though required at the start of the cycle, as the drum is evacuated the negative pressure inside the drum may eventually provide adequate force to sustain the seal. Still, the mechanical compression is required to initiate the process.

FIG. 6 is a simplified wiring diagram showing the low voltage 120/240 VAC motor and control wiring.

Operation FIGS. 1, 2, 4, and 6

Initially, an operator would open the door and lift the door to a position which it will maintain. The operator would then load the drum with wet clothes or other fabrics from a washing machine, close the door and use the locking handle 34 to seal the door. (The hinges 32 are slotted so that the door can move to the drum uniformly when the drum begins to experience negative pressure.) The operator would choose the proper position of the selector switch 45, depending upon whether the load may contain metal (zippers, buttons, rivets, etc.). Then, the operator would push the (momentary contact) start switch 46 to begin the drying cycle. The microwave generator is not switched on until the minimum vacuum is achieved, measured at the vacuum switch 41, indicating that the drum door is adequately sealed.

FIG. 1 shows the overall flow of air through the rotating drum and vacuum pump assembly. Outside (room) air initially flows through an adjustable valve 12 which is designed in conjunction with the flow orifices inside the drum and the size of the vacuum pump 17 to provide the expected flow rate. However, adjustment of the valve must be limited to preclude any significant vacuum in the microwave generator canister. Air then flows through the inside pipe of the dual flow rotary union 13 and into the drum assembly. The dielectric breakdown voltage associated with high voltage connections, although there are several different modes, varies with reduced pressure in a manner similar to that predicted by Paschen's Law. Consequently, the canister 23 that contains the high voltage microwave generating components, shown in FIG. 2 along with other drum internal parts, is designed to operate near atmospheric pressure to avoid breakdown and arcing. Details of the parts inside the canister are shown in FIG. 3. The canister and main drum 15 cavity are separated by a small diameter critical flow orifice(s) that allows the design air flow to drop from near-atmospheric pressure to the significant vacuum in the drum. The minimum air flow is that required to maintain the magnetron tube temperature below the maximum acceptable temperature for proper operation. Beyond that, the air flow is designed to provide an acceptable drying time. Other pressure drops in the system include that across the filter 16 orifices and the friction losses in the piping. However, these losses are designed to be small. The vacuum pump 17 is designed to provide the design air flow rate at the desired drum vacuum (absolute pressure), which in turn is selected to provide a maximum saturated water temperature in the drum. The proper flow rate could be attained without the external valve 12, but including the valve provides some flexibility in the design.

Inside the drum, air is contacted with fabric which is tumbling as the drum rotates, accentuated by the drum baffles 25. Although three are shown, this number may be greater. Microwaves directed from the wave guide 22 fill the drum and are absorbed by water in the fabric before and after microwaves are reflected by the grounded metal inside surfaces of the drum. The drum can be made of various metals, but aluminum is a good choice for a fabric dryer to provide high thermal conductivity, resistance to water/air corrosion, and to minimize the weight of the unit. The drum must be designed for the anticipated operating vacuum. Design for full vacuum will provide maximum flexibility in any future vacuum pump substitution.

As air exits the drum it flows across a filter material to remove lint, such material supported by a metal housing 26 drilled with many small diameter holes designed to provide minimal pressure drop at the design air flow rate, with diameter at a small fraction of the wavelength of the microwaves generated. From the filter housing 26 air flows through the inside pipe of the opposite side rotary union 13 and then to the vacuum pump 17 inlet. From the pump discharge, compressed (approximately atmospheric pressure) air flows past the moisture sensor 42 and to the outer pipe of the same side dual flow rotary union 13. Through the annular area of the double-pipe axle of the rotary drum, the air flows into and through the drum jacket 28, from the jacket to the annular area of the opposite side double-pipe axle, and then transitions from rotating axle to the stationary discharge pipe through the same rotary union that conveys inlet air and rotating low voltage supply wires. The jacket area must exclude that required to accommodate the door. This will require abrupt changes in direction of the spiral path that directs the flow from the inlet to the outlet side of the jacket.

When a new drying cycle is initiated, the vacuum pump must provide an acceptable minimum vacuum at the vacuum switch 41 (indicating that the door seal is acceptable) before microwave heating can begin. Because the initial air flowing through the vacuum pump may be relatively dry room air, the final low moisture switch 42 must be bypassed briefly so that the heating can begin and moisture laden air can make its way to the moisture switch. Microwave drying will continue until the moisture sensor detects sufficiently low moisture to indicate the potential buildup of charge on metal objects (depending upon the selector switch position). At this point, energy from microwaves is replaced by that from room air heated by an electric resistance element 39, controlled by an in-line temperature switch 40, while the air flow rate and vacuum remain essentially unchanged.

Hot air vacuum drying continues until the moisture sensor detects a low moisture level that indicates the load is acceptably dry. At this point all power would be interrupted except that the (now open) final low moisture switch will be bypassed during that part of the final revolution required to bring the drum door to the normal load/unload position. A mechanical proximity switch 47 is held closed by the drum except at a single position of the drum. This means that once per revolution the proximity switch will open briefly. As long as the moisture switch is made, the opening of the proximity switch has no effect.

When the drum stops, the operator would open the locking mechanism and raise the door to the sustaining open position and manually unload the dryer. The operator could stop the drying process at any time by pushing the (momentary contact) stop switch 48.

Alternative Embodiment

An additional embodiment would employ the same basic design but would not include the electric resistance heater 39 and the second moisture switch 42 set point. This embodiment would be used to dry other solids, such as pharmaceuticals or fine chemicals. In this case, the lack of metal objects would eliminate the arcing potential. To eliminate the possibility of corrosion and chemical contamination, 316L stainless steel would be a better choice than aluminum for drum construction. The door assembly of the first embodiment is replaced with a welded flange and bolted butterfly valve with conductive elastomeric seal. The flange diameter would accommodate installation and service of the microwave generator canister. With the butterfly valve full open, it would be possible to replace the filter media. Depending upon the nature of the solid chemical being dried, the media inside the filter may differ as required to keep fine solids out of the vacuum pump and piping. This industrial dryer designed for gravity discharge may be elevated to accommodate the height of the product container.

Advantages

From the above description, a number of advantages of the dryer apparatus are evident:

-   -   (a) Energy that would otherwise be wasted from microwave         generation, air heating, water vaporization, and vacuum pump         compression is at least partially recovered via conduction and         convection inside the drum and then from condensation and         conduction through the jacket.     -   (b) Microwave energy is absorbed almost exclusively by water and         drying temperature is minimized. Besides energy efficiency, this         low temperature drying may be helpful for synthetic fabrics or         temperature sensitive solids.     -   (c) Microwave sealing is accomplished without complex traps.     -   (d) A simple control function allows an operator to dry clothes         that may contain metal objects without arcing.

Conclusions, Ramifications, and Scope

This dryer apparatus is a relatively simple machine that can be constructed using readily available components and uncomplicated fabrications that lend themselves to mass production. There will be some challenges. For example, the double pipe drum axle center line must coincide with the drum center line within a very small tolerance to make the drum rotate without deflection. And, the overall dimension from the rotary contacts 11 on one end to the vacuum pump suction piping on the other must be such that the unit could be placed in a typical residential laundry room. This consideration does not apply to the industrial application.

If such a unit should become widely used, the energy savings could offset a higher cost, and could have ramifications at the grid level that will be helpful to all. More sophisticated appliances are now more likely to be widely accepted by energy conscious consumers. The relatively rapid low temperature drying will have both energy and time saving advantages that will be interesting to industrial users who need to dry high value temperature sensitive pharmaceuticals and fine chemicals.

Although the description above contains a number of specificities, these should not be construed as limiting the scope of the embodiments but as merely providing illustrations of these embodiments. For example, the door of the dryer designed for residential laundry could have a variety of shapes and closure mechanisms. The ends of the drum could be formed shape rather than flat. The butterfly valve suggested for loading and unloading of an industrial dryer could be another type of valve with large diameter opening—perhaps an iris valve, if one could be designed to seal under vacuum. And, the transition from the drum to valve flange could be a shape other than cylindrical to encourage gravity discharge of non-free flowing solids.

Thus the scope of the embodiments should be determined by the appended claims and their legal equivalents, rather than by the examples given. 

I claim:
 1. An apparatus for drying clothes or other solids including those containing some metal objects comprising: a. A metal drum designed for external pressure with a sealing access door and heat transfer jacket with internal baffles and means for connecting external piping to and from said jacket and an end centered double pipe axle providing conduits for fluid flow in and out of said drum and in and out of said jacket and providing for rotation of said drum. b. Double pipe rotary sealing units connected to said drum axle providing for connection of said pipes to stationary piping. c. A cylindrical metal canister located inside said drum and attached to one end of said drum and open to said drum inlet pipe and removable from said drum and providing a pressure vessel wherein high voltage microwave generator components are mounted and said canister is sealed from said drum cavity, including microwave antenna, except through one or more orifices of predetermined size avoiding microwave feedback and providing near atmospheric pressure inside said canister at design air flow rate through said canister and said drum while said drum pressure is less than one half of atmospheric pressure. d. A set of rotary contacts providing connection of stationary wires conveying residential single phase alternating voltage to wires rotating with and inside said drum axle providing power to said high voltage microwave generator components inside said canister. e. An electric motor and means for rotating said drum. f. An electric motor and vacuum pump to provide a predetermined rate of air flow through said canister and said drum and to convey that air and its moisture content to discharge piping at a pressure higher than atmospheric pressure while maintaining a predetermined negative gauge pressure inside said drum cavity. g. An electric resistance heater mounted inside said stationary inlet piping providing the design maximum inlet air temperature for said drum. h. Electric manual and automatic controlled switches that allow an operator to select a drying process that will either both turn off the microwave generator at a predetermined outlet air moisture content and simultaneously turn on said resistance heater or not depending upon the nature of the load being dried, while allowing the drying process to terminate automatically at some predetermined final moisture content of said outlet air in either case or to be terminated by said operator or manually set timer switch.
 2. The apparatus of claim 1 designed for drying various industrial solids including pharmaceuticals and fine chemicals wherein said drum cylindrical section transitions to a pipe flange and said flange is connected to a valve for loading and unloading and wherein said electric manual and automatic controls do not include provision for switching from microwave drying to hot air drying.
 3. A method of drying clothes or other solids comprising irradiation with microwaves under reduced pressure until a preset moisture level is achieved and drying energy switches to conventional electric resistance heating for those loads that may contain metal objects inside a rotating drum with a drum jacket providing energy recovery from air discharged from the drum.
 4. A method of drying pharmaceuticals or other fine chemicals or other industrial solids comprising irradiation with microwaves under reduced pressure inside a rotating drum with a drum jacket providing energy recovery from air discharged from the drum. 